In general, light emitting diodes which are formed of a compound semiconductor, such as GaAs, InP, GaN, etc., are widely used as a rear light source for traffic signals, outdoor advertising displays, liquid crystal displays, telephone keypads. Such light emitting diodes using compound semiconductors are incompatible with other photoelectric devices using silicon-based semiconductors. Therefore, silicon-based light emitting diodes that do not use compound semiconductors have been suggested. Furthermore, silicon-based light emitting diodes can be fabricated at low costs, and CMOS technologies can be used to fabricate silicon-based light emitting diodes, and silicon-based light emitting diodes can be efficiently integrated.
However, due to the indirect transition characteristic of silicon semiconductors, the light extraction efficiency of a silicon-based light emitting diode is lower than that of a compound semiconductor-based light emitting diode. For this reason, there are difficulties in practically using and commercializing silicon-based light emitting diodes developed so far.